U.S. patent application number 09/881057 was filed with the patent office on 2002-01-03 for receiver and method for avoiding intersymbol interference in a high speed transmission system.
Invention is credited to Tore, Andre.
Application Number | 20020001355 09/881057 |
Document ID | / |
Family ID | 7891324 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001355 |
Kind Code |
A1 |
Tore, Andre |
January 3, 2002 |
Receiver and method for avoiding intersymbol interference in a high
speed transmission system
Abstract
The present invention relates to a receiver for a high speed
transmission system and a method for receiving a signal on a
receiving side of a transmission system wherein a first signal is
transmitted over a transmission path. Said first signal comprises a
plurality of data symbols which are successively transmitted, in
front of each being a first prefix for avoiding an interference
between said successively transmitted data symbols. The receiver
according to the present invention comprises a filter having a pass
characteristic such that said first signal can pass, a buffer for
buffering said first signal, means for generating a second prefix
for each first prefix in front of each of said plurality of data
symbols replacing means for replacing said first prefix by said
second prefix, said second prefix respectively having a length
longer than that of said first prefix to be replaced. The invention
finds application in DMT XDSL systems.
Inventors: |
Tore, Andre; (Alvso,
SE) |
Correspondence
Address: |
NIXON & VANDERHYE P.C.
1100 North Glebe Road, 8th Floor
Arlington
VA
22201
US
|
Family ID: |
7891324 |
Appl. No.: |
09/881057 |
Filed: |
June 15, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09881057 |
Jun 15, 2001 |
|
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PCT/EP99/09197 |
Dec 16, 1998 |
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Current U.S.
Class: |
375/350 |
Current CPC
Class: |
H04L 27/2607 20130101;
H04L 27/2647 20130101 |
Class at
Publication: |
375/350 |
International
Class: |
H04B 001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 1998 |
DE |
198 58 106.8 |
Claims
1. A receiver for a high speed transmission system for receiving a
first signal (B) over a transmission path (30), said first signal
(B) comprising a plurality of data symbols (50a, 51a, 52a) which
are successively transmitted, in front of each being a first prefix
(50b, 51b, 52b) for avoiding an interference between said
successively transmitted data symbols (50a, 51a, 52a), said
receiver comprising: means (33) for generating a second prefix (53,
55) for each first prefix (50b, 51b, 52b) in front of each of said
plurality of data symbols (50a, 51a, 52a); and replacing means (34)
for replacing said first prefix (50b, 51b, 52b) by said second
prefix (53, 55), said second prefix (53, 55) respectively having a
length longer than that of said first prefix (50b, 51b, 52b) to be
replaced, characterized by a buffer (32) for buffering said first
signal; a filter (35) having a pass characteristic such that said
first signal can pass comprising a first filter portion (60) having
a finite impulse response introducing zeros in the transfer
function of said filter (35) and a second filter portion (61)
having an infinite impulse response introducing poles in the
transfer function of said filter (35). wherein said first filter
portion (60) is arranged in front of said replacing means (34) with
respect to said transmission path (30), and said second filter
portion (61) behind thereof.
2. A receiver according to claim 1, characterized by receiving a
second signal (A) over said transmission path (30) simultaneously
with said first signal (B), said first and said second signal
having different frequency bands, and said pass characteristic of
said filter (35) being such that said second signal (A) cannot pass
said filter (35).
3. A receiver according to claim 1 or 2, characterized in that for
a given data symbol (50a, 51a, 52a), said second prefix (53, 55) is
a part of said given data symbol.
4. A receiver according to claim 1, 2 or 3, characterized by said
buffer (32) comprising a first buffer portion (41) and a second
buffer portion (42); a switching means (40) for switching said
transmission path (30) to said first and second buffer portions
(41, 42) such that successively transmitted ones of said plurality
of data symbols (50a, 51a, 52a) are alternately buffered in said
first and second buffer portions (41, 42).
5. A receiver according to one of claims 1 to 4, characterized in
that, said means (33) for generating a second prefix generates said
second prefix (53, 55) with a length corresponding to a parameter
derived from a impulse response of said filter (35) such that an
interference of successive ones of said plurality of data symbols
(50a, 51a, 52a) of said first signal (B) caused by transients of
said filter (35) is avoided.
6. A receiver according to one of the claim above, characterized in
that, said transmission path (30) is a telephone line and said
second signal is a telephone service or an ISDN service.
7. A receiver according to one of claims 1 to 6, characterized in
that it is part of a modem.
8. A method for receiving a signal (B) on a receiving side of a
transmission system, said signal comprising data symbols (50a, 51a,
52a) and a first prefix (50b, 51b, 52b) in front of each data
symbol (50a, 51a, 52a) for avoiding an interference of successively
transmitted data symbols (50a, 51a, 52a), comprising the following
steps: receiving said signal on the receiving side (S1); buffering
said received signal (S2); generating a second prefix for each
first prefix in front of each of said data symbols (S3); replacing
said first prefix by said second prefix, said second prefix having
a length longer than that of said first prefix to be replaced (S4);
filtering said signal, wherein said first prefix has been replaced
by said second prefix, by means of a filter having a pass
characteristic such that said signal can pass (S5), including
firstly filtering said signal with a first filter portion (60)
having a finite impulse response (FIR) introducing zeros in the
transfer function of said filter (35) before replacing said first
prefix (51b, 51b, 52b) and secondly filtering said signal (B)
wherein said first prefixes (50b, 51b, 52b) have been replaced by
said second prefixes (50a, 51a, 52a) by means of a second filter
portion (61) having an infinite impulse response (IIR) introducing
poles in the transfer function of said filter (35).
9. A method according to claim 8, characterized in that for a given
data symbol (50a, 51a, 52a), said second prefix (53, 55) is
generated by using a part of said given data symbol.
10. A method according to claim 8 or 9, characterized in that said
buffering of said received signal (B) is performed by switching
said transmission path (30) to a first and a second buffer portion
(40, 41) such that successively received data symbols (50a, 51a,
52a) are alternately stored in said first and second buffer portion
(40, 41).
11. A method according to one of claims 8 to 10, characterized in
that said second prefix (53, 55) is generated with a length
corresponding to a parameter derived from an impulse response of
said filter (35) such that an interference of successive data
symbols (50a, 51a, 52a) of said signal (B) caused by transients of
said filter (35) is avoided.
Description
[0001] The present invention relates to a receiver according to the
preamble of claim 1 and to a method for receiving a signal on a
receiving side of a transmission system, said signal comprising a
plurality of data symbols having a prefix for avoiding an
interference of successively transmitted data symbols caused by
transients filtered by the transmission path.
TECHNOLOGICAL BACKGROUND
[0002] ADSL (Asymmetrical High Speed Digital Subscriber Line) and
VDSL (Very High Speed Digital Subscriber Line) systems, usually
referred to as xDSL-systems use an ordinary telephone line to
transmit digital data at high speed. This is done in overlay on the
analogue POTS (Plain Old Telephone Service) service. Thanks to
xDSL, telephone companies can re-use most of their installed wiring
for the introduction of new services. The xDSL-system uses a higher
frequency band than an ordinary telephone service or an ISDN
(Integrated Services Digital Network) service.
[0003] The basic structure of such a XDSL system is shown in FIG.
9. Reference number 1 in FIG. 9 designates a telephone exchange,
which is connected to a transmission path 3 via a first LP (low
pass) Filter 2. A first XDSL modem 4 is connected on the telephone
exchange side to said transmission path 3 via a first HP (high
pass) Filter 5. Said first LP-Filter 2 and said first HP-Filter 5
form a first splitter filter 6 which is used for separating the
telephone or ISDN service from a signal transmitted over said
transmission path 3 by said XDSL system. Said transmission path 3
can be e.g. a twisted-pair subscriber line.
[0004] On the subscriber side, there is provided a subscriber
terminal 7 such as a telephone which is connected to said
transmission path 3 via a second LP Filter 8. A second XDSL modem 9
is connected to said transmission path 3 via a second HP Filter 10.
Said second LP Filter 8 and said second HP Filter 10 form a second
splitter filter 11 used for separating the telephone or ISDN
service from the signal transmitted by said xDSL system.
[0005] The relationship of the frequency bands used for the
telephone or ISDN services and a VDSL system is shown in FIG. 10.
Reference character A indicates the frequency band used for the
transmission of speech in the telephone service or used by the ISDN
service. In the case of the telephone service, the frequency band A
ranges from 0 Hz to 3 kHz and in the case of the ISDN service from
0 Hz to 160 kHz. The frequency band used by the VDSL system is
indicated with reference character B. As shown in FIG. 9, this
frequency band ranges from 300 kHz to 10 MHz.
[0006] The characteristics of said first and second splitter
filters 6 and 7 have to be such that the respective LP Filter 2 or
8 has such characteristics that said telephone or ISDN service may
pass, but the frequency band B used by the xDSL system is
suppressed. The characteristic of the respective HP Filter 5 or 10
has to be such that the frequency band B used for the data
transmission at high speed by said xDSL systems may pass, but said
frequency band A used by said telephone or ISDN service is
suppressed.
[0007] A modulation scheme standardized for ADSL (defined in the
ANSI standard on ADSL, entitled "Asymmetric Digital Subscriber Line
(ADSL) Metallic Interface Specification", published by the American
National Standards Institute Inc. (ANSI)) and suggested for VDSL is
called DMT (Discrete Multi Tone) modulation. In DMT systems, data
symbols are modulated on a plurality of carriers (preferably a set
of 256 carriers with equidistant frequencies). This modulation is a
QAM (Quadrature Amplitude Modulation) which is carried out on said
plurality of carriers at the same time, which are then added
together. The demodulation can be implemented as an FFT (Fast
Fourier Transformation). The modulation can be implemented as an
IFFT (Inverse Fast Fourier Transformation). The output from one
IFFT calculation is called a DMT-symbol. A detailed description of
the principles of DMT is given in J. A. C. Bingham, Multicarrier
modulation for data transmission: an idea whose time has come, IEEE
Communications Magazine, May 1990, pp. 5-14.
[0008] A problem occurring in such DMT-systems with respect to the
HP-Filters used in such xDSL-systems is that the HP-Filters 5 and
10 used in the xDSL-signal path must be of a high order to ensure a
clear separation from the telephone or ISDN service. These filters
have a long impulse response that may cause the successively
transmitted DMT symbols to interfere with one another. A DMT symbol
input to the HP filter causes transients in said filter. If the
transients have not died out before the next DMT symbol is input,
these transients produce distortions in the next symbol that let
one DMT-symbol interfere with the successive one (intersymbol
interference).
[0009] EP 0 802 649 A1 suggests a solution for compensating for
intersymbol interference caused by transients filtered in the
transmission path. A prefix and/or suffix and/or symbol extension
is inserted in front of each DMT-symbol before transmission of the
DMT-symbol over the transmission path. To fully compensate for
intersymbol interference, the length of this prefix and/or suffix
and/or symbol extension equals at least the length of the equalized
channel impulse response of the respective transmission path.
[0010] EP 0 725 509 A1 suggests to use a prefix having p-bits (p
being an integer) whose values are equal to the values of p-bits at
the end of the respective data symbol.
[0011] FIG. 11a shows such a DMT-symbol with a prefix which is
arranged in front of said DMT-symbol. The prefix is a copy of the
end of the DMT-symbol.
[0012] FIG. 11b shows two successively sent DMT-symbols 20 and 22,
respectively having a prefix 21 and 23. In front of each of said
prefixes 21 and 23, there are transients filtered in the
transmission paths. The cause of these transients will be explained
with more detail with reference to FIG. 12.
[0013] FIG. 12 shows two of the plurality of carriers in a
DMT-signal. The two carriers 30 and 31 have different frequencies.
The first carrier 30 has a first frequency f.sub.1 and the second
carrier 31 has a second frequency f.sub.2. Reference number 24
relates to a first DMT-symbol on said first carrier 30 with a
prefix 25, followed by a second DMT-symbol 26 with a prefix 27.
[0014] Reference number 27 relates to a third DMT-symbol with a
prefix 28, followed by a fourth DMT-symbol 29 with a prefix 30 on
the second carrier 31. As should be understood from FIG. 12, all
carriers will be continuous from the beginning of the cited prefix
to the end of the respective DMT-symbol by selecting the end of the
respective DMT symbol as the respective prefix.
[0015] It is furthermore indicated in FIG. 12 that the transients
are caused by discontinuities in the carriers 30 and 31. FIG. 12
shows transients caused between the end of the DMT-symbols 24 and
27 and the beginning of the respective prefix 27 and 30 of the
following DMT-symbol 26 and 29. By setting the length of the
respective prefixes in front of said DMT-symbols 26 and 29 to a
length with respect to the impulse response of the transmission
path, it is thereby possible to avoid intersymbol interference
since the transients caused by such a discontinuity will die out
before the DMT-symbol 26 and 29, carrying the respective data,
starts.
[0016] With an increase of the transmission rate, the requirements
for the HP-Filter at the receiving side become more stringent,
implying longer impulse responses for the HP-Filter especially on
the receiving side. To avoid intersymbol interference, the lengths
of the prefixes have to be extended.
[0017] An extension of the prefixes causes a reduction of the data
throughput capacity of the transmission system since the
transmission time required for the transmission of these prefixes
increases. Thereby, the transmission time needed for the
transmission of data and the resources occupied by the transmission
are increased. Accordingly, in the design of such a xDSL-system
transmitting DMT-symbols, there is a problem that the transmission
rate is limited and a compromise or trade-off has to be found
between the transmission rate, the requirements for the HP-Filter
at the receiving side and the length of the prefixes in front of
the DMT-signals.
[0018] FIG. 13 shows the impulse response of a VDSL-signal path
including HP-Filters. In case of the transmission path shown in
FIG. 13, it takes nearly 450 samples until the impulse response has
completely died out. A compromise in this case would be that the
prefix has a length of 32 to 128 samples.
[0019] EP 0 829 988 A2 describes symbol synchronization and
sampling frequency adjustment in an OFDM receiver wherein the pulse
response of a radio channel is determined and a guard interval is
set in the receiver such that it covers the most significant
components of the pulse response. A slow and monotonous temporal
shift of the pulse response between measurement rounds indicates an
error in the sampling frequency which is thereafter compensated
for.
[0020] WO 97/30531 A1 describes a transmitter arranged to insert a
guard space in data bursts to be transmitted and being able to
adjust the duration, that is the length of the inserted guard
space. A receiver may be adapted to produce a guard space of a
minimum duration necessary to prevent data loss caused by delay
spread.
[0021] EP 0 929 172 A1, published on Jul. 14, 1999, describes a
multicarrier modulation system with variable symbol rates wherein
in fallback modes an increased guard time is provided for a better
delay spread tolerance and increased symbol length provides
improved signal to noise performance. As described in claim 3 of
document D5, a guard time is interposed between successive ones of
the symbols, while the length of the guard time is greater for
modes with a greater value of K.
[0022] It is, therefore, an object of the present invention to
provide a receiver according to the preamble of claim 1 and a
method for receiving a signal on a receiving side of a transmission
system which allow to reduce intersymbol interference of data
symbols received successively from a transmission path without
reducing the throughput capacity of the transmission system.
[0023] This object is solved by a receiver with the features of
claim 1 and a method with the features of claim 8.
[0024] On the receiving side a received first prefix in front of
each data symbol is replaced by a second, longer prefix, which is
locally generated on the receiving side of the transmission system.
This locally generated second prefix allows that the transients
caused by the filter on the receiving side have enough time to die
out before the data symbol comprised in the signal sent over the
transmission path is input to the filter.
[0025] This allows that an impulse response of a filter of a high
order on the receiving side of the transmission system can be
locally compensated without reducing or influencing the
transmission rate on the transmission path.
[0026] The meaning of "to replace" in the context of the present
application is not limited to a complete replacement of the first
prefix by the second prefix. That is, the term "to replace" as used
in the present application includes to replace only a part of the
first prefix by the second prefix and not only the whole first
prefix. In other words, it is possible to replace e.g. only the
first or last 10 samples of the first prefix by the second prefix
or to replace a suitable number of samples in the middle of the
first prefix.
[0027] In an advantageous embodiment of the present invention, said
second locally generated prefix is formed from a part of the data
symbol in front of which said second prefix is used to replace said
first prefix.
[0028] Due to this arrangement, it can be ensured that the carriers
are continuous from the beginning of the second prefix to the end
of the respective data symbol.
[0029] Further improvements and embodiments of the present
invention become apparent from the dependent claims.
[0030] The invention may be more fully understood in conjunction
with the accompanying drawings.
[0031] FIG. 1 shows a first embodiment of a receiver for a high
speed transmission system according to the present invention.
[0032] FIG. 2 is a flow-chart showing the operation of the receiver
of FIG. 1.
[0033] FIG. 3 shows a second embodiment of the receiver for high
speed transmission system according to the present invention.
[0034] FIG. 4 shows a chart for explaining the operation of the
receiver of FIG. 3.
[0035] FIG. 5a shows an example of a second order HP-Filter
comprising a FIR-Filter and an IIR-Filter.
[0036] FIGS. 5b-5d show the frequency responses of the FIR-Filter
portion, the IIR-Filter portion and of the whole filter shown in
FIG. 5a.
[0037] FIG. 6 shows a third embodiment of the receiver for high
speed transmission system according to the present invention.
[0038] FIG. 7 shows a fourth embodiment of the receiver for a high
speed transmission system according to the present invention.
[0039] FIG. 8 shows an embodiment of a xDSL modem according to the
present invention.
[0040] FIG. 9 shows the basic structure of a xDSL system.
[0041] FIG. 10 shows the frequency bands used by VDSL Systems and
telephone or ISDN services.
[0042] FIGS. 11a and 11b show DMT symbols with prefixes.
[0043] FIG. 12 shows two carriers of a DMT symbol.
[0044] FIG. 13 shows an example of a total impulse response of a
VDSL signal path including sharp HP-filters.
[0045] In the following, a first embodiment of a receiver for a
high speed transmission system according to the present invention
is described with respect to FIG. 1. In this high speed
transmission system, a signal is transmitted over a transmission
path 30 from a sending side (not shown) to the receiver 31. Said
signal comprises a plurality of data symbols which are sent over
said transmission path 30 one after the other. In front of each of
said data symbols, there is a first prefix. This first prefix is
for avoiding an interference between the successively transmitted
data symbols occurring due to transients filtered by the
transmission path 30. The transmission path 30 is connected to a
receiver 31 of the receiving side of the transmission system.
[0046] The receiver 31 comprises a buffer 32 for buffering the
received signal. A means 33 for generating a second prefix is
connected to said buffer 32 and to a replacing means 34 for
replacing said first prefix sent with each said of said data
symbols in said received signal. A filter 35 is provided which is
connected to said buffer 32 and has its output 36 connected to a
decoder (not shown) for decoding the data symbols. In case, the
receiver is used in a xDSL system, said filter 35 is a HP (high
pass) filter, preferably with sharp slopes.
[0047] The operation of the receiver according to the first
embodiment described with reference to FIG. 1 is now described with
reference to FIG. 2.
[0048] In step S1 of the flow-chart of FIG. 2, the signal sent from
a sending side over the transmission path 30 to the receiver 31 is
received. The received signal is then buffered or stored in the
buffer 32 in step S2. Then, in step S3 said means for generating a
second prefix generates a second prefix for each first prefix in
front of each of said plurality of data symbols contained in said
received signal. This second prefix may have an individual length
for each of the first prefixes to be replaced, but preferably has a
fixed length for all first prefixes to be replaced based on the
characteristics such as the impulse response of the filter 35. The
generation of the second prefix is preferably done by taking a part
of the respective data symbol the first prefix of which should be
replaced, and to use this part as said second prefix to replace
said first prefix. This part of the respective data symbol can be
the end of the respective data symbol. In case said data symbol is
a digital symbol, a number of samples from the end of said digital
data symbol may be taken as second prefix such that the sample
values of said second prefix having a length of n-samples are equal
to the values of n-samples at the end of the respective data
symbol.
[0049] Said means for generating a second prefix 33 generate said
second prefix with a length corresponding to a parameter derived
from an impulse response of said filter 35. Said second prefix may
for example be generated with a length in accordance with a
parameter relating to the time needed that transients of said
filter 35 have died out to a level having no negative effect for
the following data symbol such that an interference of data symbols
sent one after another in said signal caused by transients of said
filter 35 is avoided. Said means for generating a second prefix 33
can also determine the length of said second prefix in
correspondence to a parameter relating to the time necessary that
the impulse response of said HP-Filter 35 shows a predetermined
attenuation, e.g. an attenuation of 40 dB and in correspondence
with the frequency with which said data symbols are input to said
filter 35.
[0050] Then, in step S4 the first prefix as sent with the
respective data symbol in said signal is replaced by said second
prefix generated in said step S3. It has to be noted that instead
of replacing said first prefix by said second prefix, said
replacing means 34 according to a variant of the first embodiment
of the receiver can be adapted to add said second prefix to said
first prefix or to insert said second prefix in said first prefix
to thereby extend the first prefix. In case said data symbols and
said first and second prefixes are digital signals, the sample
values of the second prefix can be used to replace suitable sample
values of the first prefix, e.g. the first 15 samples of the first
prefix, or the sample values of the second prefix can be inserted
in said first prefix at sample positions at the beginning, in the
middle or at the end of the first prefix. Thereby, the first prefix
can be either extended or replaced by a longer second prefix at the
receiving side of the transmission system, i.e. in the receiver,
without reducing the data transmission rate over the transmission
path in spite of using a filter 35 on the receiving side with an
impulse response that cannot be compensated for with the length of
the first prefix.
[0051] Then, the sent signal, wherein the first prefixes in front
of the respective data symbols have been replaced by said second
prefixes, are filtered by means of said filter 35. The filtered
signal can then be decoded by means of a decoder (not shown) or
further processed in accordance with the modulation scheme used in
the transmission system.
[0052] In case, the above-described receiver is used in a
xDSL-system as described in detail in the introductory part of this
application, said description being incorporated herewith into the
description of the invention, the signal is transmitted over a
transmission path, such as a twisted pair subscriber line
simultaneously with other services, such as e.g. a telephone
service or an ISDN (Integrated Services Digital Network) service.
In this case, the above signal comprising a plurality of data
symbols uses a different frequency band than that used by said
telephone or ISDN service. The filter 35 used in such a system has
a pass characteristic, such that a frequency band of said signal
containing the data symbols can pass, but other frequency bands
used e.g. by the telephone service or the ISDN service are
suppressed.
[0053] If the above-described receiver is used in an ADSL or VDSL
system using the DMT modulation scheme, as described in more detail
in the introductory part of the present application, said data
symbols correspond to DMT-symbols comprising a combined set of
modulated carriers, wherein e.g. 2 samples of the data to be
transmitted from the sending side to the receiving side of the
transmission system, are modulated via 4 QAM (Quadrature Amplitude
Modulation) on a first carrier, 8 samples are modulated e.g. via
256 QAM on a second carrier and so on such as described in the
above referenced EP 0 802 649 A1.
[0054] FIG. 3 shows a second embodiment of the receiver for a high
speed transmission system according to the present invention. The
receiver according to the second embodiment of the present
invention comprises a first switching means 40 for connecting a
first buffer 41 or a second buffer 42 to the transmission path 30.
The first buffer 41 and the second buffer 42 are respectively
connected to means for generating a second prefix 33 and replacing
means 34 for replacing said first prefix by said second prefix,
which are similar to the first embodiment described with reference
to FIG. 1. The first buffer 41 can be connected to a decoder (not
shown) via a first HP Filter 43 (HP 0 in FIG. 3) by means of a
second switching means 45. Said second buffer 42 can also be
connected to said decoder (not shown) via a second HP Filter 44 (HP
1) by means of said second switching means 45.
[0055] The operation of the second embodiment of the receiver
according to the present invention is now described with reference
to FIG. 4.
[0056] The first line in FIG. 4 shows the received signal, which
was sent over the transmission path 30 and input to the receiver.
The received signal shown in the first line of FIG. 4 comprises
three data symbols, namely symbol 0, symbol 1 and symbol 2,
indicated with reference numbers 50a, 51a and 52a, respectively. In
front of each of said data symbols 50a, 51a and 52a there is a
respective first prefix cp, respectively indicated with reference
numbers 50b, 51b, 52b.
[0057] As already explained in detail in the introductory part of
this application, transients are caused at the beginning of each of
said first prefixes 50b, 51b and 52b in said transmission path. In
FIG. 4, it is assumed that the first prefixes 50b, 51b and 52b are
sufficiently long for ensuring that transients of said transmission
path 30 have already died out before said symbols 50a, 51a and 52a
are transmitted and that there is no intersymbol interference
caused in said transmission path 30.
[0058] Reference number 53 indicate small amplitude-time diagrams
representing the impulse response of said first and second
HP-Filters 43 and 44 (HP 0 and HP 1) for showing a dimensional
comparison of the length of the first prefixes 50b, 51b and 52b, as
contained in said signal transmitted over said transmission path 30
to the impulse response of the first and second HP-filters 43 and
44.
[0059] Said first switching means 40 switches said transmission
path to said first buffer 41 and to said second buffer 42
alternately, such that successively transmitted data symbols 50a,
51a and 52a are alternately stored in said first buffer 41 and said
second buffer 42. In other words, if the switching state shown in
FIG. 3 is assumed, the data symbol indicated with reference number
50a in FIG. 4 is stored in said first buffer 41 together with the
respective first prefix 50b. Then, said switching means 40 is
switched to said second buffer 42, and the following symbol, namely
the symbol indicated with reference number 51a in FIG. 4, is stored
in said second buffer 42 together with the respective first prefix
51b. Then, said first switching means 40 is switched back to said
first buffer 41 and the successively received data symbol, namely
data symbol 52a in FIG. 4, is stored in said first buffer 41
together with the respective first prefix 52b.
[0060] Then, said means 33 for generating a second prefix generates
a second prefix 54, 55 in the same manner as described with
reference to FIG. 1 for of said first prefixes 50b, 51b and 52b
stored in the first and the second buffer 41 and 42.
[0061] The replacing means 34 replaces said first prefixes 50b, 51b
and 52b in the first and second buffer 41 and 42 by said second
prefix 54, 55. It has to be noted, that, as described with
reference to FIG. 1, said replacing means 34 can also be adapted to
extend said first prefix 50b, 51b and 52b by inserting said second
prefix 54, 55 into said first prefix 50b, 51b and 52b or to add
said second prefix 54, 55 to the respective first prefix 50b, 51b
and 52b.
[0062] As shown in line 2 of FIG. 4, the data symbol 50a is input
to the first HP-Filter 43 with a second (or extended) prefix 54
(ECP) in front. The first prefix 50b has been replaced by said
second prefix 54. It may be noted that in the context of the
present invention, the term "replace" is to be understood broadly
as relating to a suitable change in the prefix. The term does not
imply that the first prefix has to be removed completely or the
like, because the replacement can simply consist in an extension of
the first prefix as indicated above. Naturally, it is also possible
that the first prefix is removed completely before a new prefix is
added.
[0063] As indicated with the hatched portion in data symbol 0
indicated with reference number 51a, and the hatched portion of
said second prefix 54, a part of said data symbol 0 is used as said
second prefix 54.
[0064] As shown with the impulse response of the HP filters HP 0
and HP 1 (reference numbers 43 and 44 in FIG. 3) illustrated with
the amplitude-time diagrams 53, the second prefix 54 is long enough
to ensure that the transient of the HP filter HP 0 has died out
before the data symbol 50a is inserted to the HP filter HP 0.
[0065] As shown in line 3 of FIG. 4, the second symbol 1 carrying
the reference number 51a is input to the second HP-Filter HP 1
carrying the reference number 44.
[0066] As indicated with the hatched portions in the data symbol
51a and the second prefix 55 in front of said data symbol 51a,
parts of said data symbol 51a are used as said second prefix
55.
[0067] By alternately connecting said HP filters HP 0 and HP 1 to a
decoder (not shown) by the second switching means 45, data symbols
50a and 51a are successively input to the decoder (not shown) and
can be further processed in accordance with the respective
modulation scheme.
[0068] FIG. 5a shows an example of an HP-Filter, which can be used
as HP-Filters HP 0 and 1 in FIG. 3 (reference numbers 43 and 44) or
as Filter 35 in FIG. 1. For the sake of clarity of illustration,
this HP-Filter is of a low order. However, as is apparent for the
skilled person, HP-Filters having a higher order can be used in
order to better adapt the filter characteristics to the
transmission path.
[0069] The high pass filter shown in FIG. 5a comprises a first
filter portion constituted by an FIR-Filter 60 and a second filter
portion constituted by an IIR-Filter 61. The FIR-Filter 60 and the
IIR-Filter 61 are connected in series with each other.
[0070] The input of the FIR filter 60 is connected to a first
one-sample delay element 62. The output of said first one-sample
delay element 62 is connected to the input of a second one-sample
delay element 63. The input of the FIR filter 60 is further
multiplied by a coefficient a.sub.0 by means of a first multiplier
64 and added to the output of the first one-sample delay element
62, which has been multiplied with a coefficient a.sub.1 by means
of a second multiplier 65, by means of a first adder 66. The output
of the first adder 66 is added to the output of the second
one-sample delay element 63, which has been multiplied with a
coefficient a.sub.2 by a third multiplier 67 by means of a second
adder 68. The first and second one-sample delay elements 62 and 63,
the first, second and third multipliers 64, 65 and 67 and the first
and second adders 66 and 68 constitute the FIR-Filter 60.
[0071] The output of the FIR-Filter 60, namely the output of the
adder 68 is input to a third adder 69, whose output is the output
of the IIR-Filter 61 and whose output is input to a third
one-sample delay element 70. The output of the third one-sample
delay element is input to a fourth one-sample delay element 71. The
output of the third one-sample delay element 70 and the fourth
one-sample delay element 71 are respectively multiplied with
coefficients a.sub.3 and a.sub.4 by means of a fourth multiplier 72
and a fifth multiplier 73, and then added by means of a fourth
adder 74. The output of the fourth adder 74 is then added to the
output of FIR-Filter 60 by means of said third adder 69.
[0072] The third and fourth one-sample delay elements 70 and 71,
the fourth and fifth multipliers 72 and 73 and the third and fourth
adder 69 and 74 constitute said IIR-Filter 61.
[0073] The FIR-Filter 60 can be described by the following equation
with n being the time index, x(n) being the input of the Filter and
y(n) being the output.
y(n)=a.sub.0x(n)+a.sub.1x(n-1)+a.sub.2x(n-2);
[0074] The IIR-Filter 61 can be described by the following equation
with n being the time index, y(n) being the input of the Filter and
v(n) being the output.
v(n)=y(n)+a.sub.3v(n-1)+a.sub.4v(n-2);
[0075] The total impulse response of the high pass filter shown in
FIG. 5a is obtained by convoluting the above two equation.
[0076] Accordingly, the total impulse response is longer than each
of the individual impulse responses of the FIR- and the IIR-Filter
as indicated with the above equations.
[0077] FIG. 5b shows the frequency response of the FIR-Filter 60,
and FIG. 5c shows the frequency response of the IIR-Filter 61. The
total frequency response of the HP filter shown in FIG. 5a is shown
in FIG. 5d.
[0078] As apparent from FIGS. 5a-5d, the FIR-Filter 60 is a
nonrecursive filter, which introduces zeros in the transfer
function of the HP filter shown in FIG. 5a and attenuates the low
frequencies. The impulse response thereof is finite.
[0079] The IIR-Filter 61 is recursive and introduces poles in the
transfer function of the HP filter shown in FIG. 5a and compensates
in this arrangement for the frequency response of the FIR-Filter
60, and flattens the frequency response in the pass band. The
impulse response of the IIR-Filter 61 is infinite.
[0080] In case the HP filter in FIG. 5a is realized as a digital
filter, the FIR-Filter 60 and IIR-Filter 61 can be separated.
[0081] FIG. 6 shows the third embodiment of the receiver according
to the present invention, using a HP filter comprising a recursive
and a nonrecursive filter portion, such as the one described with
reference to FIGS. 5a-5d.
[0082] FIG. 6 shows a third embodiment of the receiver according to
the present invention which has a similar arrangement as the
receiver according to the second embodiment described with
reference to FIG. 3, except that the HP filters HP 0 and HP 1,
indicated with reference numbers 43 and 44 in FIG. 3, are replaced
by a HP filter having two filter portions, namely a recursive
filter portion and a nonrecursive filter portion. This HP filter is
constituted depending on the switching state of the first switching
means 40 by a first filter portion HP FIR, indicated with reference
number 80, which is arranged in front of said replacing means 34
with respect to the transmission path, and one of second filter
portions HP IIR 0 and HP IIR 1, indicated with reference numbers 81
and 82 which are arranged behind said replacing means 34.
[0083] In FIG. 6, the first filter portion 80 (HP FIR) is arranged
in front of said first switching means 40 and connected to said
transmission path 30. Thereby, said first filter portion 80 (HP
FIR) is used for both of said second filter portions 81 and 82 (HP
IIR 0, HP IIR 1) for respectively forming a HP filter having a pass
characteristic, such that the signal comprising said plurality of
data symbols with a prefix in front of each can pass.
[0084] Said first filter portion 80 (HP FIR) is a nonrecursive
filter such as the one indicated with reference number 60 in FIG.
5a and has a finite impulse response. Said second filter portions
81 and 82 (HP IIR 0 and HP IIR 1) are recursive filters such as the
one indicated with reference number 61 in FIG. 5a and respectively
have an infinite impulse response.
[0085] The first filter portion 80 (HP FIR) filters non-periodic
low frequency disturbances from the signal received over the
transmission path 30. Said second filter portions 81 and 82 (HP IIR
0 and HP IIR 1) respectively compensate for the frequency response
of the first filter portion 80 (HP FIR) and respectively flatten
the frequency response of the whole HP filter (constituted by HP
FIR and HP IIR 0 or HP FIR and HP IIR 1, respectively) in the pass
band.
[0086] With this arrangement an unwanted low frequency disturbance
of said telephone or ISDN services can be separated from the signal
containing said data symbols.
[0087] FIG. 7 shows a fourth embodiment of the receiver according
to the present invention. The receiver according to the fourth
embodiment of the present invention is similar to the receiver
according to the third embodiment of the present invention,
described with reference to FIG. 6, except that said second
switching means 45 is arranged between the outputs of said first
buffer 41 and said second buffer 42 for alternately switching said
outputs to a second filter portion 83 (HP IIR). The output of the
second filter portion 83 is connected to a decoder (not shown)
formed in accordance with the respective modulation scheme.
[0088] Said second switching means 45 are arranged to alternately
connect said first and said second buffers 41 and 42 to said second
filter portion 83 (HP IIR). Due to this arrangement, only one
second filter portion 83 (HP IIR) has to be arranged allowing to
reduce the complexity of the receiver and to reduce the
manufacturing costs thereof.
[0089] It has to be noted that said first switching means 40 and
said second switching means 45 switch with a certain frequency
which is synchronized with the frequency of said data symbols in
said received signal, if said data signals contained in said
received signal are periodic. In case that the data symbols are not
periodic, said switching means are controlled by means of a data
symbol detecting means (not shown). This data symbol detecting
means detects the data symbols or the first prefixes in front of
said data symbols in said received signal and controls the
switching of said first and second switching means 40 and 35 such
that the switching is performed alternately after each data
symbol.
[0090] FIG. 8 shows an embodiment of a modem according to the
present invention. This modem is for sending and receiving a signal
over a transmission path 30. The sent or received signal comprises
a plurality of data symbols which are successively transmitted. In
front of each of said data symbols is a first prefix for avoiding
an interference between successive data symbols caused by
transients of said transmission path 30. This signal preferably
uses a first frequency band which is different from a second
frequency band used by other signals transmitted simultaneously
over said transmission path 30. In case of a high speed xDSL
system, as discussed in detail in the introductory part of the
description, said signal is transmitted in a frequency band
starting from 300 kHz up to 10 MHz, and other signals are telephone
or ISDN services which are transmitted in a frequency band ranging
from 0 to 3 kHz (POTS) or 0 to 160 kHz (ISDN).
[0091] For the following description of the modem in FIG. 8 it is
assumed that the modem is a DMT xDSL modem. However, it has to be
noted that--as apparent to the skilled person--the present
invention can also be applied to other transmission systems wherein
a signal comprising successively transmitted data signals is sent
over a transmission path.
[0092] Reference number 90 in FIG. 8 designates an IFFT (Inverse
Fast Fourier Transformation) transformation means which converts a
set of modulated carriers from an encoder from frequency domain to
time domain. This combined set of modulated carriers is a data
symbol and is called, in case of a DMT xDSL modem, a DMT-symbol. A
first prefix adder 91 is provided for adding a first prefix to each
of said data symbols with a length corresponding to the impulse
response of the transmission path 30 for avoiding an interference
between successively transmitted data symbols caused by transients
of said transmission path 30.
[0093] A parallel-to-serial converter 92 connected to the output of
said prefix adder 90 generates a serial data stream by serializing
successive data symbols. The serialized data symbols are then
transformed into an analogue signal by a digital-to-analogue
converter 93 connected to an output of said parallel-to-serial
converter 92. The thereby generated signal is then applied to the
transmission path 30 via switching means 94.
[0094] Optionally, a digital HP-Filter (not shown) can be provided
between said parallel-to-serial converter 92 and said
digital-to-analogue converter 93, and an analogue HP-Filter can
optionally be provided between said digital-to-analogue converter
93 and said switching means 94 for removing disturbances from the
signal applied to said transmission path 30.
[0095] Said switching means 94 connects the sending branch of the
modem comprising said IFFT transformation means 90, said prefix
adder 91, said parallel-to-serial converter 92 and said
digital-to-analogue converter 93 to the transmission path 30 if a
signal is to be sent from the encoder to said transmission path 30
and connects the transmission path 30 to a receiving branch of said
modem, which will be described in the following under the
assumption that a signal is sent from a sending side (not shown) to
the modem.
[0096] If a signal comprising a plurality of data symbols is
received over said transmission path 30, said switching means 94 is
connecting said transmission path 30 to the receiving branch of the
modem. Said received analogue signal is firstly digitized in an
analogue-to-digital converter 95, and then input to a time domain
equalizer 96, which digitally filters the received data symbols to
compensate for intersymbol interference on the transmission path
30.
[0097] A first HP filter 97 is connected to the output of the time
domain equalizer 96 for removing low frequency disturbances which
are not periodic compared to the data symbol length from the
received signal. This first HP filter 97 is preferably a
nonrecursive HP filter having a finite impulse response. This first
HP filter 97 forms a HP filter system together with a second HP
filter 101, which is a recursive filter having an infinite impulse
response. The nonrecursive HP filter 97 introduces zeros in the
transfer function of the HP filter system and attenuates the low
frequencies. The recursive HP filter 101 introduces poles into the
transfer function of the HP filter system and compensates for the
frequency response of the first HP filter 97.
[0098] Preferably, the FIR-Filter 60 described with reference to
FIG. 5a, is used as first HP filter 97 and the IIR-Filter 61
described with reference to FIG. 5a as said second HP filter
101.
[0099] The output of the first HP filter 97 is input to buffer
means 98 for buffering the received signal. Then, means for
generating a second prefix 99 are generating a second prefix having
a length longer than that of the respective first prefix in front
of each of said data symbols.
[0100] Said second prefix is preferably generated with a length
corresponding to a parameter derived from the impulse response of
the filter system constituted by said first HP filter 97 and said
second HP filter 101, such that an interference of successive ones
of said plurality of data symbols of the received signal caused by
transients of said filter system is avoided. This parameter may
relate--as already indicated with reference to the first
embodiment--to the time until the amplitude of the impulse response
of the filter system is reduced to a certain threshold, or shows an
attenuation of e.g. 40 dB, after a Dirac impulse was input to the
filter system. This parameter may also relate to a time (or time
period) after the input of a data symbol after that the transients
caused by the data symbol have no adverse effect on the
successively input data symbol.
[0101] The impulse response is the output time function of the
respective system, in this case the filter system, being applied
with a Dirac impulse (or unit impulse) having an impulse integral
of 1 at the time t=0 at the input.
[0102] Replacing means 100 for replacing said first prefix by said
second prefix generated in said means 99 for generating a second
prefix respectively replace the first prefixes in front of said
data symbols by said second prefix.
[0103] The output of said replacing means 100 is input to said
second HP filter 101. The output of the second HP filter 101 is
input to a serial-to-parallel converter 102, which can further be
adapted to remove the respective second prefix from the data
symbols. The output of said serial-to-parallel converter 102 is
then input to a FFT (Fast Fourier Transformation) transformation
means 103, which converts the data symbols from the time domain to
frequency domain which are then output to a suitable decoder.
[0104] Optionally, the receiving branch of said modem may further
comprise an analogue high pass filter (not shown) which is arranged
between said switching means 94 and said analogue-to-digital
converter 95 for removing disturbances from the received
signal.
[0105] In a variant of said modem, said filter system constituted
by said first HP filter 97 and said second HP filter 101 is
replaced by a single filter, e.g. of the kind of filter 35
described with reference to FIGS. 1 and 2 which is arranged behind
said replacing means 100.
[0106] In a further variant, said buffer means 98 comprises a first
buffer portion and a second buffer portion similar to the first
buffer 41 and the second buffer 42 described with reference to
FIGS. 3, 4, 6 and 7 and said modem further comprises a first buffer
switching means similar to the first switching means 40 described
with reference to FIGS. 3, 4, 6 and 7. Then, the modem further
comprises second buffer switching means similar to the second
switching means 45 described with reference to FIGS. 3, 4, 6 and 7,
either arranged in front of the second HP filter 101 similar to the
arrangement shown in FIG. 7, or behind two of said second HP
filters 101 similar to the arrangement shown in FIG. 6 or behind
two high pass filters such as the HP filters HP 0 and HP 1
described with reference to FIG. 3 if instead of said first HP
filter 97 and said second HP filter 101, one part filters such as
the HP filters HP 0 and HP 1 are used, similar to the arrangement
shown in FIG. 3.
[0107] It is noted that, in other embodiments, the above features
may be used in combination with each other.
[0108] Although the present application has been described with
reference to specific embodiments, it has to be noted that the
embodiments and examples given above are for illustrative purposes
only and for a better understanding of the present inventions and
by no means intended to limit the scope of the present invention.
Other modifications are clearly possible for a person of ordinary
skills in the art without departing from the scope of the present
invention as defined in the following claims.
[0109] Reference signs are intended for a better understanding and
shall not limit the scope.
* * * * *